To mount a semiconductor chip on a substrate, it is necessary to electrically connect an electrode formed on the semiconductor chip and a conductor formed on the substrate. The most general method as the above connection method uses the wire bonding method in which a chip is turned upward and secured to a substrate, and an electrode formed on the chip and a conductor pad formed on the substrate are connected to each other by a thin metal wire (gold wire is mainly used). This method is effective for a semiconductor chip having a relatively simple structure even at present and it is a leading method of connection methods for connecting a semiconductor chip with a substrate.
However, when the structure of a chip is complex, it is necessary to bond a great number of wires on a chip and thus, the cost remarkably increases or a lot of time is required for bonding. Therefore, a wireless bonding method was developed which connects a semiconductor chip onto a substrate without using a bonding wire. The flip chip method and the TAB method are typical ones of the wireless bonding method.
The flip chip method is a method for directly connecting a conductive pad formed on a substrate with an electrode formed on a semiconductor chip by turning the chip downward and bringing the chip turned downward into contact with the substrate. The TAB method is a method for connecting a long conductor (hereafter referred to as a lead) formed on a TAB tape with a semiconductor chip and thereafter, cutting a tape to leave only a part of the lead and connecting an end of the lead with a conductive pad formed on a substrate.
Any one of these wireless bonding methods realizes electrical connection through a metal bump formed on an electrode of a semiconductor chip. For example, in the case of the flip chip method, a metal bump is formed on an electrode of a semiconductor chip to keep the interval between the semiconductor chip and a substrate constant.
Connection between a metal bump and a substrate is performed by using solder formed on a conductive pad of the substrate as a brazing filler material and melting the material. Moreover, in the case of the TA method, a metal bump is formed on an electrode of a semiconductor chip and serves as a contact point with a lead formed on a TAB. In the case of the TAB method, a fusible metal layer made of, for example, tin formed so as to cover a lead serves as a brazing filler for connecting a metal bump and the lead.
Problems of the flip chip method are described below in detail. Gold is mainly used as a metal bump. A metal bump is connected with a substrate by previously forming solder serving as a brazing filler for connection on the substrate like a bump by a method such as injection, aligning the solder bump with a gold bump formed on a chip, and thermally melting (reflowing) the solder bump. The solder bump used in the above case is fusible solder made of indium, tin, and lead.
However, the above conventional method has the following problems.
Firstly, when using a gold bump for the flip chip method, the solder composition of a solder bump formed at the substrate side is restricted to a certain condition. For example, it is impossible to use the most general binary-system solder made of tin and lead. This is because, when a junction is formed by using the above composition, gold-tin eutectic is formed on the interface between a gold bump and a solder layer and thereby, the connection reliability is deteriorated. In this case, the connection reliability denotes that the present connection state can be maintained because a joint is physically and chemically stable under the working condition. Secondly, a recess with a depth of several microns is easily formed on the top of a gold bump and thereby, a void is produced at the interface between the gold bump and a solder layer when a chip is mounted.
The above described first point further includes the following two problems. Firstly, the cost of indium-based solder increases. Moreover, it is necessary to previously form a solder bump by a method such as injection and thereby, the process cost increases. In the field of semiconductor mounting in which reduction of fabrication cost is a supreme subject, increase of unnecessary cost must be prevented.
The above described second point is more minutely described below by referring to FIG. 1. FIG. 1 is a schematic view of a cross section when making connection by using a bump with a recess at its top. As shown in FIG. 1, small voids are produced at the top of the bump because bubbles present in the recess cannot escape at the time of solder reflow. A joint including a lot of voids of the above type has a problem in its reliability.
Then, problems of the TAB method are described below.
FIGS. 2A and 2B show schematic views of the TAB method. To make connection by the TAB method, a lead at the TAB side (normally made of copper) is plated with a fusible metal such as tin to make connection with a bump formed on a semiconductor chip. In FIGS. 2A and 2B show a lead is formed on a TAB tape (not illustrated) and extends in the direction vertical to the paper surface. At least a part of the lead is plated with a fusible metal to serve as a brazing filler for connection with the bump formed on the semiconductor chip. The bump is connected with the lead by making them contact each other and heating and contact-bonding them.
However, even in the case of this method, it is necessary to plate a lead with a fusible metal layer serving as a brazing filler material because a bump normally uses gold. Therefore, increase of process cost cannot be avoided.
As described above, though the flip chip method and the TAB method has a superior aspect as the wireless bonding method, it is necessary to further improve the cost and reliability. The present invention solves these problems by improving the structure of a metal bump formed on a semiconductor chip.
The first object of the present invention is to provide a metal bump formed on a semiconductor chip to connect the semiconductor chip with a substrate, a metal bump structure having a preferable compatibility with many types of solder for connection according to the flip chip method, and its manufacturing method.
The second object of the present invention is to provide a metal bump formed on a semiconductor chip to connect the semiconductor chip with a substrate, a metal bump structure having a smooth top and preventing the connection reliability from deteriorating for connection according to the flip chip method, and its manufacturing method.
The third object of the present invention is to provide a metal bump formed on a semiconductor chip to connect the semiconductor chip with a lead formed on a TAB tape without using a fusible metal layer on a lead, and its manufacturing method.
The Forth object of the present invention is to solve the above problems on flip chip mounting and TAB mounting by providing a substitute structure of a gold bump according to the prior art and a method for forming the substitute structure.